Michael J. Withford

Method for determination of the volume of material ejected as molten droplets during visible nanosecond ablation

A novel method is presented for determining the volume of molten material ejected from a substrate as a result of visible pulsed-laser ablation. A 100-microm-wide pulsed-laser light sheet (tau approximately 5 ns, lambda = 532 nm) was used in conjunction with a CCD camera to provide high-speed cross-sectional images of single-pulse ablation of aluminum with a visible nanosecond laser source. Computational analysis of the two-dimensional gray-scale images was used to determine the total volume of material ejected from the substrate in the form of molten droplets. Ablation with dual-wavelength (511- and 578-nm) pulses of 30-ns duration was characterized under various fluence conditions (0-25 J cm(-2)), allowing a quantitative threshold for explosive melt ejection in aluminum to be established at approximately 10 J cm(-2). The temporal evolution of the ejected material showed that, for an incident fluence of approximately 40 J cm(-2), molten-droplet ejection commenced at approximately 400 ns and ceased after approximately 2 micros.

Pulsed Laser Pump Wavelength Influence on the Efficiency and Stability of the Alexandrite Laser

Pulsed laser pumping was investigated for a range of monochromatic wavelengths from 532nm to 671nm. Shortening pump wavelength was associated with increased non-radiative energy decay, reduced efficiency and chaotic spectral, temporal and transverse modal characteristics

Effects of high pulse repetition frequency on the ablation of polymers

Measurement of ablation depths and etch rates are presented for PETG and polyimide (kapton) under pulsed UV(255nm) laser processing for a pulse repetition frequency (prf) range of 750Hz-15kHz using a frequency doubled copper vapour laser and a fixed fluence of 0.59 J/cm2. Results show that the material removal rate (μm/sec) scales approximately linearly with pulse repetition frequency, and there appears to be no attenuation of the ablating laser beam by the ejected material plume for this fluence. The etch rate (μm/pulse) increases slightly (~20%) for high prf (15kHz), an effect which is attributed to cumulative heating of the sample.Measurement of ablation depths and etch rates are presented for PETG and polyimide (kapton) under pulsed UV(255nm) laser processing for a pulse repetition frequency (prf) range of 750Hz-15kHz using a frequency doubled copper vapour laser and a fixed fluence of 0.59 J/cm2. Results show that the material removal rate (μm/sec) scales approximately linearly with pulse repetition frequency, and there appears to be no attenuation of the ablating laser beam by the ejected material plume for this fluence. The etch rate (μm/pulse) increases slightly (~20%) for high prf (15kHz), an effect which is attributed to cumulative heating of the sample.

Photonic crystal based circuitry fabricated using laser-assisted processes

Recently, photonic crystals (PCs) have gained interest due to their ability to influence the propagation of light [1]. This paper describes a simple and novel approach for the fabrication of micron and nanometer scale photonic crystal based circuitry with an arbitrary degree of control. Laser ablation using a femtosecond Ti:Sapphire [2] laser is used to form a template from which the photonic crystal device is castRecently, photonic crystals (PCs) have gained interest due to their ability to influence the propagation of light [1]. This paper describes a simple and novel approach for the fabrication of micron and nanometer scale photonic crystal based circuitry with an arbitrary degree of control. Laser ablation using a femtosecond Ti:Sapphire [2] laser is used to form a template from which the photonic crystal device is cast

Fabrication of Tilted Fibre Grating for Medical Applications

Fibre gratings are used in a multitude of telecommunication systems but their application to other fields has been somewhat overlooked. In recent years, the application of fibre gratings to other fields such as sensor systems has been growing. Optical fibre devices are also increasingly being used in medicine for endoscopes and keyhole surgery. In these cases optical fibres are employed in an end-firing configuration. However, for surgical applications such as those used in the treatment of atrial fibrillation, a side-firing system is desired. At Macquarie University, tilted fibre gratings have been fabricated for such an application. The characteristics of tilted fibre gratings fabricated using a frequency doubled copper vapour laser will be presented.Fibre gratings are used in a multitude of telecommunication systems but their application to other fields has been somewhat overlooked. In recent years, the application of fibre gratings to other fields such as sensor systems has been growing. Optical fibre devices are also increasingly being used in medicine for endoscopes and keyhole surgery. In these cases optical fibres are employed in an end-firing configuration. However, for surgical applications such as those used in the treatment of atrial fibrillation, a side-firing system is desired. At Macquarie University, tilted fibre gratings have been fabricated for such an application. The characteristics of tilted fibre gratings fabricated using a frequency doubled copper vapour laser will be presented.

Characterisation of waveguides written in bulk materials with femtosecond laser pulses

With the goal of fabricating photonic devices for optical networking applications, significant attention has recently been directed to the use of femtosecond laser pulses for inscribing optical components inside transparent materials. In particular, it has been demonstrated that a tightly focussed Ti: Sapphire laser pulse can induce a change in the refractive index inside bulk glass without damage. Using this mechanism it is possible to fabricate a range of waveguide devices; such as beam splitters, couplers and interferometers, inside a wide range of materials without the need for linear absorption. At CUDOS @ Macquarie we have fabricated and characterised a number of structures in a variety of glasses. A review of this research will be presented.With the goal of fabricating photonic devices for optical networking applications, significant attention has recently been directed to the use of femtosecond laser pulses for inscribing optical components inside transparent materials. In particular, it has been demonstrated that a tightly focussed Ti: Sapphire laser pulse can induce a change in the refractive index inside bulk glass without damage. Using this mechanism it is possible to fabricate a range of waveguide devices; such as beam splitters, couplers and interferometers, inside a wide range of materials without the need for linear absorption. At CUDOS @ Macquarie we have fabricated and characterised a number of structures in a variety of glasses. A review of this research will be presented.

Laser based fabrication of micro and nano-structures using sacrificial layers

When using ultra-short laser pulses it is possible to machine areas smaller than the focal spot by restricting the threshold light intensity to the centre of the spot. However, even smaller and cleaner features can be achieved by utilising a sacrificial material on top of the layer being machined. In particular, we have found that features can be reduced by up to a factor of two and the heat-affected-zone by a factor of three in polymers. These results have implications for improved fabrication of fluid and gas delivery systems and optical devices such as photonic crystals and related devices.When using ultra-short laser pulses it is possible to machine areas smaller than the focal spot by restricting the threshold light intensity to the centre of the spot. However, even smaller and cleaner features can be achieved by utilising a sacrificial material on top of the layer being machined. In particular, we have found that features can be reduced by up to a factor of two and the heat-affected-zone by a factor of three in polymers. These results have implications for improved fabrication of fluid and gas delivery systems and optical devices such as photonic crystals and related devices.

Active waveguides written in rare-earth doped materials with femtosecond laser pulses

Optical waveguides can be fabricated inside various glasses by using tightly focussed femtosecond laser pulses to induce a change in the refractive index. Researchers have shown this direct-write technique has the potential to generate both planar and three-dimensional photonic devices for optical networking applications inside a wide range of materials. In particular, waveguides can be written in Erbium-doped phosphate glass and demonstrate a net gain when used as an active element in a standard waveguide amplifier. We report results of a detailed study optimising the beam delivery configurations and fabricating photonic waveguides and devices in both passive and active glasses.Optical waveguides can be fabricated inside various glasses by using tightly focussed femtosecond laser pulses to induce a change in the refractive index. Researchers have shown this direct-write technique has the potential to generate both planar and three-dimensional photonic devices for optical networking applications inside a wide range of materials. In particular, waveguides can be written in Erbium-doped phosphate glass and demonstrate a net gain when used as an active element in a standard waveguide amplifier. We report results of a detailed study optimising the beam delivery configurations and fabricating photonic waveguides and devices in both passive and active glasses.

Direct-write microcutting and micropatterning of polymers using high prf UV copper lasers

Phenomenal market growth is predicted for laser micromachining applications in the next decade as microelectronic and medical industries begin to rely on microprocessing as a critical manufacturing process. In this paper we discuss techniques used for micropattterning by direct write machining, presenting how the groove quality and depth are effected by source fluence and sample speed for both highly and poorly absorbing polymers.Phenomenal market growth is predicted for laser micromachining applications in the next decade as microelectronic and medical industries begin to rely on microprocessing as a critical manufacturing process. In this paper we discuss techniques used for micropattterning by direct write machining, presenting how the groove quality and depth are effected by source fluence and sample speed for both highly and poorly absorbing polymers.

Ablation threshold and etch rate measurements in high-speed ultra-violet (uv) micro-machining of polymers with uv-copper vapour lasers

Material removal rates (etch rates) are given for the ultra-violet (uv) ablation of polyimide, mylar and PETG by uv-Copper Vapour Lasers (CVLs) operating at 255 nm and at pulse repetition rates (prfs) from 4.25 to 20 kHz for fluences in the range 0.08 to 2.2 J/cm2. These results and the extrapolated thresholds are compared with those obtained by excimer laser ablation at similar wavelengths and fluences, and with the ablation model of Srinivasan, Smrtic and Babu. Qualitative results for PMMA, which has lower absorption at this wavelength, are also presented. Possible thermal effects of ablation at these high prfs are discussed.Material removal rates (etch rates) are given for the ultra-violet (uv) ablation of polyimide, mylar and PETG by uv-Copper Vapour Lasers (CVLs) operating at 255 nm and at pulse repetition rates (prfs) from 4.25 to 20 kHz for fluences in the range 0.08 to 2.2 J/cm2. These results and the extrapolated thresholds are compared with those obtained by excimer laser ablation at similar wavelengths and fluences, and with the ablation model of Srinivasan, Smrtic and Babu. Qualitative results for PMMA, which has lower absorption at this wavelength, are also presented. Possible thermal effects of ablation at these high prfs are discussed.